A driving apparatus for a twin-screw extruder is known from DE 101 01 255 A1 (corresponding to US 2001/008 372 A1). The driving apparatus comprises a gearbox driven, at the input end, by a main motor and an auxiliary motor while the output end thereof is connected to the screw shafts of the twin-screw extruder. The gearbox comprises a superposition gear mechanism configured as a planetary gear mechanism, a distribution gearbox and a speed change gearbox. The speed change gearbox serves to ad-just the rotational speed or speed of the screw shafts. To change the speed, one pair of gears is replaced by another pair of gears having a different gear ratio.